The fine structure of the mouse parathyroid gland both in the normal and in the hypofunctional condition induced by the intraperitoneal injection of calcium chloride was observed with the electron microscope.The parenchymal cells of the mouse parathyroid gland are classified into three types according to the ultrastructural characteristics, i. e. the light chief cells, the dark chief cells, and the intermediate chief cells. The light chief cells (the inactive chief cells) are characterized by an electron lucid cytoplasm, poor cytoplasmic components and a few secretory granules. The dark chief cells (the active chief cells) have moderately electron dense cytoplasmic matrix, prominent Golgi apparatus, numerous mitochondria and secretory granules. The intermediate chief cells, which are predominant in number in the normal mouse, show a transitional cytoplasmic structure between these two types.The secretory granules are electron dense spherical bodies of 150-200mμ in diameter composed of finely granular material. In the well-developed Golgi apparatus, granules of different densities as well as typical secretory granules are observed, thus suggesting the maturing process of the secretory granules. The secretory granules, which are usually located at the peripheral region of the cytoplasm, are occasionally attached to the plasma membrane suggesting the secretion into the extracellular space.Various types of inclusion bodies are seen in the parenchymal cell. Occurrence of multivesicular bodies and their possible significance are reported.Occasional cilia with the usual ultrastructure are observed in the parenehymal cells of the mouse parathyroid gland.The capillary endothelium of this gland is of fenestrated type.In the parathyroid gland of the calcium administered animals, the light chief cells increase in relative number so that they become the predominant cell type of the parenchymal cell. The secretory granules decrease markedly after long term administration of calcium chloride. In the parathyroid of the mouse administered with 0.4ml of 2% calcium chloride for 14 days, swollen prosecretory granules appear in the cytoplasm of the gland cell. The swollen prosecretory granules may be considered to appear as the result of the inhibition of the maturating process of the secretory granules.From the comparison between the parathyroid gland of the normal and of the experimental mouse, three types of the gland cell may be considered to represent different functional stages of the parenchymal cell.